1. Field of the Invention
The present invention relates to an imaging apparatus including a lens apparatus and a camera apparatus, such as a video camera or the like. More particularly, the present invention relates to a focusing operation and focusing control and especially, an imaging apparatus for broadcasting.
2. Description of the Related Art
A conventional imaging apparatus such as a video camera or the like is equipped with an automatic focus detection/focusing control (hereinafter referred to as “AF control”) function. As an AF control system, a contrast-type automatic focus detection system has been mainly used. In this contrast-type automatic focus detection system, a signal corresponding to sharpness (contrast) of an object is extracted from an imaged video signal to be used to carry out a focus detection operation of an imaging optical system.
FIG. 6 is a block diagram showing one example of a configuration of an imaging apparatus. An AF control operation using the contrast-type automatic focus detection system will be described with reference to FIG. 6.
A lens apparatus 401 is removably attached to a camera body 413. The lens apparatus 401 includes a first fixed lens unit 402, a second lens unit (zoom lens) 403, a diaphragm 404, a third fixed lens unit 405, and a fourth lens unit (focus lens) 406. The second lens unit 403 is movable for carrying out zooming. The fourth lens unit 406 is equipped with both a focus adjusting function and a compensation function for compensating for the movement of a focal plane caused by zooming.
Light from an object is focused on an imaging plane of an image sensor 414 in the camera body 413 via each lens unit and the diaphragm 404 of the lens apparatus 401. The formed object image is converted into an electrical signal by the image sensor 414. The electrical signal from the image sensor 414 is supplied as an image signal to an image processing unit 415.
In the image processing unit 415, the input image signal is sampled and held. The sampled and held image signal is amplified to have a predetermined level by AGC (automatic gain control), and is then converted into a digital signal by A/D (analog-to-digital) conversion. Further, the digital signal is converted into a standard television signal. The standard television signal is output to a video output terminal 416.
Then, the video signal is input to a video signal input terminal 407 of the lens apparatus 401 via a cable 417, and is then supplied to an AF evaluation value generating unit 408. In the AF evaluation value generating unit 408, a high-frequency component, which varies according to a focusing condition, is extracted from the video signal by filtering or the like, and a sharpness evaluation value (an AF evaluation value) is generated using the high-frequency component. The AF evaluation value is output to an AF drive control unit 409.
The AF drive control unit 409 determines a driving direction of the focus lens 406 as to whether an in-focus point is located on a far distance side or a close distance side with respect to the current position of the focus lens 406. In order to determine the driving direction, the AF drive control unit 409 drives a motor 412 so as to minutely drive (wobble) the focus lens 406. Then, the AF drive control unit 409 determines an in-focus direction based on a variation of the AF evaluation value output from the AF evaluation generating unit 408 in each position of the focus lens 406.
Thus, the AF drive control unit 409 moves the focus lens 406 at a predetermined speed such that the AF evaluation value output from the AF evaluation generating unit 408 reaches a peak value. After the AF evaluation value passes over the peak value, the AF drive control unit 409 reverses a driving direction of the focus lens 406. Then, the AF drive control unit 409 minutely drives the focus lens 406 such that the AF evaluation value becomes maximum (see Japanese Patent Application Laid-Open No. 9-065184). Furthermore, the AF drive control unit 409 controls motors 410 and 411 to drive the zoom lens 403 and the diaphragm 404.
However, the above-described conventional AF control system has the following disadvantages in the case where a video signal input from the video signal input terminal 407 has text or character information superimposed thereon.
1) When the superimposed information exists in a focus detection area, the superimposed information becomes a high-frequency component. Therefore, an in-focus direction cannot be determined based on the AF evaluation value obtained from the AF evaluation value generating unit 408 even if the focus lens 406 is driven between the far distance side and the close distance side (in a hill-climbing fashion) to determine the in-focus direction.
2) As for the AF evaluation value obtained from the AF evaluation value generating unit 408, even if the focus lens 406 is driven in a whole area (in a whole scanning fashion), the superimposed information becomes a high-frequency component as described above. Therefore, a suitable AF evaluation value corresponding to positions for an object and the focus lens 406 cannot be obtained. Thus, a suitable in-focus position cannot be obtained. As a result of this, an AF control operation may stop in a defocused state of an object, or the AF control operation may not be completed. Thus, the superimposed information may hinder the AF control operation.
3) If the superimposed information is information superimposed in a blinking manner at intervals of a predetermined number of fields or frames of a video signal, blinking of the superimposed information may be recognized as a variation of an object even when the object is in an in-focus condition. In that case, an unnecessary AF control operation may be activated.